To maximize the comfort of the vehicle occupants, various forms of vehicle noise reduction systems have been proposed. Typically, the noises in the passenger compartment are captured by a microphone, and canceling sound created by inverting the phase of the captured noises is emitted from a loudspeaker to cancel the noises by the canceling sound. See JP2013-112139A, for instance.
In such a vehicle noise reduction system, as the road noise and the engine noise are the primary sources of noises, the signals representing noises from such sources of noises may be used as a reference signal which is to be taken into account by the noise control unit of the vehicle noise reduction system. In particular, the road noise is created by the rolling contact of the tires with the road surface and transmitted to the passenger compartment via the suspension system.
In designing a vehicle noise reduction system, the sound transmission property of the passenger compartment is represented by a transfer function, and the canceling sound is created based on this transfer function. The above mentioned prior art proposes to modify the transfer function depending on the traveling speed of the vehicle.
The suspension system is typically provided with rubber bushes in an effort to improve the ride quality of the vehicle. To improve the ride quality, soft rubber bushes are desirable. However, in order to improve the handling of the vehicle particularly during a cornering, the suspension system is desired to be stiff. Therefore, it has been proposed to use variable elastic bushes that can increase the stiffness of the bushes during cornering or other instances where a stiff suspension is desired, and otherwise decreases the stiffness to ensure a favorable ride quality. See JP2013-116641A, for instance.
Softer bushes are beneficial also in insulating the road noises. When the bushes are stiff, road noises (in particular high frequency components thereof) are transmitted to the passenger compartment with a relatively small attenuation. In conjunction with such a suspension system using variable stiffness bushes, it was discovered that the conventional noise canceling system provides a relatively poor performance in canceling noises when the stiffness of the bushes is increased.
This may be attributed to the fact that the transfer function used by the noise canceling system is unable to represent the actual sound environment under all acoustic conditions, primarily due to the limited positioning and number of the microphones and loudspeakers.